The present invention relates to a lithium-ion secondary cell comprising an Mn cathode electroactive material, the cathode electroactive material, and a process for producing the active substance. The cell has high initial discharge capacity, and the discharge capacity barely reduces even when charging and discharging are repeated at 60xc2x0 C.
As a cathode electroactive material which is employed in a non-aqueous electrolyte secondary cell (hereinafter may be referred to as xe2x80x9cnon-aqueous secondary cellxe2x80x9d), lithium manganate of spinel structure (hereinafter may be referred to as xe2x80x9cspinel LiMn2O4xe2x80x9d) has currently been studied due to its advantages: it has an abundant resource base, can be produced at low cost, and is very safe. However, the electric capacity of the cells is drastically reduced at 60xc2x0 C. or higher, and thus the aforementioned materials are unsatisfactory as cathode electroactive materials employable in a non-aqueous secondary cell. For example, in a non-aqueous secondary cell comprising such a material serving as a cathode electroactive material, when charging and discharging are repeated 500 times, the discharge capacity of the cell is at most 90 mAh/g. Therefore, there has been a demand for further improvements to non-aqueous secondary cells.
Japanese Patent Application Laid-Open (kokai) No. 7-262984 discloses a cell comprising, as a cathode electroactive material, lithium manganate coated with an Li2MnO3 layer which is produced through heat treatment of a mixture of a lithium compound and LiMn2O4 at 400-1,325xc2x0 C. In addition, Japanese Patent Application Laid-Open (kokal) No. 10-172571 discloses a cell containing lithium manganate having a two-layer structure which is produced by soaking lithium manganate having a spinel structure in a solution containing an Li ion or Mn ion, and then heating the resultant lithium manganate at 300-1, 200xc2x0 C.
In view of the foregoing, the present invention provides a non-aqueous secondary cell exhibiting excellent cycle characteristics, which comprises a negative electrode containing lithium as an active substance, a non-aqueous electrolyte, and a positive electrode comprising, as an active substance, a spinel composite oxide comprising lithium, manganese, and oxygen, wherein the discharge capacity is maintained at 95 mAh/g or more after charging and discharging of the cell are repeated 500 times at 60xc2x0 C. The present invention also provides a cathode electroactive material which enables production of a cell exhibiting excellent cycle characteristics, and a process for producing the active substance.
The present invention provides:
(1) a cathode electroactive material comprising a composite oxide comprising xcex2-MnO2 and a spinel oxide predominantly comprising lithium, manganese, and oxygen;
(2) a cathode electroactive material comprising a composite oxide according to (1), wherein the xcex2-MnO2 is comprised on the surface of the composite oxide;
(3) a cathode electroactive material comprising a composite oxide according to (1) or (2), wherein the xcex2-MnO2 is comprised in the composite oxide in an amount of about 1-13 mol %;
(4) a cathode electroactive material comprising a composite oxide according to any one of (1) through (3), wherein the spinel oxide predominantly comprising lithium, manganese, and oxygen is LiMn2O4 formed of lithium, manganese, and oxygen; or a composite oxide Li1+xMn2xe2x88x92xxe2x88x92yMyO4 (wherein xe2x88x920.1xe2x89xa6xxe2x89xa60.2, and 0xe2x89xa6yxe2x89xa60.2) in which a portion of Li or Mn in LiMn2O4 is replaced by a different element such as chromium, cobalt, aluminum, nickel, iron, or magnesium;
(5) a cathode electroactive material comprising a composite oxide according to any one of (1) through (4), wherein the composite oxide is a granule having a particle size of about 3-50 xcexcm;
(6) a cathode electroactive material comprising a composite oxide according to any one of (1) through (5), which has pores of about 30-400 xc3x85;
(7) a paste for producing an electrode, which comprises a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6);
(8) a positive electrode comprising a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6);
(9) a process for producing a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6), which process comprises acid treatment of a composite oxide comprising a spinel oxide predominantly comprising lithium, manganese, and oxygen, and heat treatment of the resultant composite oxide at a temperature of about 200xc2x0 C. or higher and lower than about 400xc2x0 C.;
(10) a non-aqueous secondary cell comprising a negative electrode comprising lithium as an active substance, a non-aqueous electrolyte, and a positive electrode comprising, as an active substance, a composite oxide comprising lithium, manganese, and oxygen, wherein the composite oxide is a cathode electroactive material comprising a composite oxide as recited in any one of (1) through (6).
The present invention will next be described in detail.
The process for producing a cathode electroactive material of the present invention comprises acid treatment of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen; and heat treatment of the composite oxide at a specific temperature; i.e., about 200xc2x0 C. or higher but lower than about 400xc2x0 C. By means of the process, a composite oxide comprising xcex2-MnO2 and an oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen can be produced. Preferably, the surface layer of the composite oxide compresses substantially of xcex2-MnO2. As used herein, the term xe2x80x9ca composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygenxe2x80x9d refers to LiMn2O4 formed of lithium, manganese, and oxygen; and a composite oxide Li1+xMn2xe2x88x92xxe2x88x92yMyO4 (wherein xe2x88x920.1xe2x89xa6xxe2x89xa60.2, and 0xe2x89xa6yxe2x89xa60.2) in which a portion of Li or Mn of LiMn2O4 is replaced by a different element such as chromium, cobalt, aluminum, nickel, iron, or magnesium. The lattice constant of the composite oxide is preferably 8.240 xc3x85 or less.
As used herein, the phrase xe2x80x9ccomprises substantially of xcex2-MnO2xe2x80x9d refers to the case in which the surface layer of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen comprises at least one lattice of xcex2-MnO2. 
In the present invention, no particular limitation is imposed either on the process for producing a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen, or the starting material of the composite oxide. For example, such a composite oxide may be produced through the following process: a mixture of a manganese compound and a lithium compound, or a mixture of a manganese compound, a lithium compound, and a compound comprising an element which can replace to manganese may be calcined at a temperature of about 300-850xc2x0 C. for at least about one hour in air or under oxygen-gas flow.
No particular limitation is imposed on the crystallinity of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen. The composite oxide may contain a non-reacted lithium compound or manganese oxide. Examples of manganese sources which may be employed as a starting material include electrolytic manganese dioxide (EMD), chemically synthesized manganese dioxide (CMD), manganese sesquioxide, trimanganese tetroxide, manganese oxyhydroxide, manganese carbonate, and manganese nitrate. Examples of lithium sources which may be employed include lithium hydroxide, lithium carbonate, and lithium nitrate. Of the aforementioned manganese sources, manganese carbonate is preferable due to its high reactivity with lithium.
In the present invention, no particular limitation is imposed on the species of acid which may be employed, so long as the acid is able to dissolve lithium and manganese on the surface of a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen. For example, a Bronsted acid may be employed. Examples of Bronsted acids include inorganic acids such as hydrochloric acid, hydrobromic acid, hydroiodic acid, hydrofluoric acid, nitric acid, amidosulfuric acid, sulfuric acid, and phosphoric acid; organic sulfonic acids such as methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, naphthalenesulfonic acid, and naphthalenedisulfonic acid; and organic carboxylic acids such as trifluoroacetic acid, trichloroacetic acid, formic acid, and oxalic acid. Of these, hydrochloric acid, nitric acid, sulfuric acid, or an organic sulfonic acid is preferable.
When a composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen is subjected to acid treatment using nitric acid, the surface layer of the oxide having a spinel structure is changed into xcex-MnO2 through the reaction represented by the following formula:
2LiMn2O4+4HNO3xe2x86x922LiNO3+Mn(NO3)2+3 xcex-MnO2+2H2Oxe2x80x83xe2x80x83(1).
In the process for producing a cathode electroactive material of the present invention, completion of conversion of LiMn2O4 having a spinel structure into xcex-MnO2 through the reaction represented by formula (1) can be confirmed by the detection of pH in the acidic solution. The conversion rate of LiMn2O4 having a spinel structure may be determined in accordance with the mol of the acid employed in acid treatment. In the process for producing a cathode electroactive material of the present invention, a composite oxide having a spinel structure, which is partially converted into xcex-MnO2 through acid treatment, is be heated at a specific temperature within a range of about 200xc2x0 C. or higher but less than about 400xc2x0 C., to thereby produce a composite oxide, the surface layer of which comprises substantially of xcex2-MnO2.
With reference to the above-described reaction, the Journal of Solid State Chemistry 39, 142-147 (1981) reported that LiMn2O4 having a spinel structure could be converted into xcex-MnO2 through treatment with a water-soluble acid solution. However, this publication does not describe or suggest a method for converting only the surface layer of the composite oxide having a spinel structure into xcex-MnO2. In addition it does not describe the composite oxide having a two-layer structure in which the surface layer of LiMn2O4 having a spinel structure is xcex-MnO2. The composite oxide having a two-layer spinel structure employed as a cathode electroactive material in a non-aqueous secondary cell is also not described. The Journal of Solid State Chemistry 39, 142-147 (1981) reports that xcex-Mno2 has a semi-stable phase and can be converted into xcex2-MnO2 which has a stable phase, through heat treatment of a sufficient duration, but this publication does not describe use of xcex2-Mno2 in the composite oxide having a two-layer spinel structure or as a cathode electroactive material in a non-aqueous secondary cell.
A cathode electroactive material which is produced through the process of the present invention is a composite oxide comprising xcex2-MnO2 and an oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen. Preferably, the surface layer of the composite oxide having a spinel structure substantially comprises xcex2-MnO2. The composite oxide is effectively employed as a material for a positive electrode in a non-aqueous secondary cell.
In the acid treatment of the surface of the spinel composite oxide predominantly comprising lithium, manganese, and oxygen, the concentration and the mol of the acid employed or the reaction time may be determined in accordance with the desired amount (mol) of xcex-MnO2. The conversion (mol %) of LiMn2O4 having a spinel structure into xcex-MnO2 is preferably about 1-13 mol % on the basis of the entirety of the spinel composite oxide, more preferably about 2-7 mol %, much more preferably about 3-mol %. When the conversion (mol %) into xcex-MnO2 is less than about 1 mol %, the intended effects are not obtained through surface treatment, whereas when the conversion is in excess of about 13 mol%, the initial discharge capacity of a non-aqueous secondary cell comprising the composite oxide is drastically and unsatisfactorily reduced.
When the surface of the composite oxide comprises only xcex-MnO2, lithium ions are taken into the oxide during discharging, and then a composite oxide having only a spinel structure is reproduced. As a result, the surface layer of the composite oxide cannot be stabilized due to the repeated charging and discharging of the cell. Therefore, in the present invention, the cathode electroactive material comprising the composite oxide having a spinel structure comprising xcex2-MnO2 in an amount of about 1-13 mol % is employed as the material for the positive electrode in the non-aqueous secondary cell.
In the non-aqueous secondary cell of the present invention, the composite oxide which is employed as the cathode electroactive material preferably comprises a xcex2-MnO2 phase in the surface layer. The composite oxide has a primary particle size of about 0.1-1 xcexcm, preferably about 0.2-0.5 xcexcm. In the present invention, the composite oxide may be granulated so as to have a primary particle size of about 3-50 xcexcm, preferably about 5-30 xcexcm.
In the present invention, the composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen can be produced as dense granules. The granules are produced through the following procedure: the composite oxide is calcined and pulverized; the thus-pulverized particles (which are primary particles or secondary particles formed through coagulation of the primary particles, and preferably having a mean particle size of about 2 xcexcm or less) are mixed with a sintering accelerator (granulation accelerator) and the resultant mixture is calcined to achieve granulation. As used herein, the term xe2x80x9cdense granulesxe2x80x9d refers to granules comprising the primary particles of the composite oxide having no or few spaces therebetween. Such dense granules may be produced by use of a sintering agent as follows.
No particular limitation is imposed on the method for mixing a sintering accelerator with the pulverized composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen. For example, the mixing may be carried out by use of a medium-stirring type pulverizing machine, a ball mill, a paint shaker, or a mixer. The pulverized composite oxide may be wet-mixed or dry-mixed with the sintering accelerator. The composite oxide may be mixed with the sintering accelerator during pulverizing of the oxide.
The sintering accelerator employed is not particularly limited, so long as it enables sintering of pulverized particles of the composite oxide predominantly comprising lithium, manganese, and oxygen which in turn allows granulation of the particles. The sintering accelerator is preferably a compound which melts at about 900xc2x0 C. or lower. For example, the compound may be an oxide which melts at about 550-900xc2x0 C. or a precursor which may be converted into the oxide; or an oxide which melts or reacts with lithium or manganese, or a compound which can be converted into the oxide. The sintering accelerator may be a compound comprising an element such as Bi, B, W, Mo, or Pb. Such compounds may be employed in combination. The sintering accelerator may be a compound comprising B2O3 and LiF, or a compound comprising MnF2 and LiF. The sintering accelerator is more preferably a compound comprising Bi, B, or W, since such a compound exerts a potent shrinkage effect.
Examples of Bi compounds include bismuth trioxide, bismuth nitrate, bismuth benzoate, bismuth hydroxyacetate, bismuth oxycarbonate, bismuth citrate, and bismuth hydroxide. Examples of B compounds include boron sesquioxide, boron carbide, boron nitride, and boric acid. Examples of W compounds include tungsten dioxide and tungsten trioxide.
The amount of sintering accelerator added to the composite oxide is about 0.0001-0.05 mol (as reduced to a metallic element in the accelerator) on the basis of 1 mol of Mn contained in the oxide. If the amount is less than 0.0001 mol, the sintering accelerator exerts no sintering-shrinkage effect, whereas if the amount is in excess of about 0.05 mol, the initial capacity of the active substance comprising the composite oxide becomes very low. The amount is preferably about 0.005-0.03 mol.
The sintering accelerator may be employed in the form of particles, or may be dissolved in a solvent to be employed in a liquid form. When the sintering accelerator is employed in the form of particles, the accelerator preferably has a mean particle size of about 50 xcexcm or less, more preferably about 10 xcexcm or less, much more preferably about 3 xcexcm or less. The sintering accelerator is preferably added to the pulverized composite oxide particles before granulation and sintering of the particles. Alternatively, after granulation of the particles, the resultant granules are impregnated with the sintering accelerator at a temperature at which the accelerator can melt, and sintering may be carried out following this step.
A method for the granulation will next be described.
Granulation may be carried out using a sintering accelerator in the form of spray granulation, flow granulation, compression granulation, or stirring granulation. Granulation may be carried out in conjunction with medium-flow drying or medium-vibration drying. Stirring granulation or compression granulation is preferably employed because secondary particles having a high density can be produced. Spray granulation is preferably employed because true-spherical granules can be produced. Examples of stirring granulation apparatuses include a vertical granulator (product of Paurec) and a SPARTAN RYUZER (product of Fuji Paudal). Examples of compression granulation apparatuses include a roller compactor (model: MRCP-200, product of Kurimoto Tekko). Examples of spray granulation apparatuses include a mobile-minor-type spray dryer (product of Ashizawaniro Atomizer).
In the present invention, no particular limitation is imposed on the size of the granules employed in the positive electrode. When the mean size of the granules is excessively large, the granules can be slightly pulverized immediately after granulation or after sintering of the particles and then subjected to classification and size regulation to thereby obtain granules of a desired size. In order to enhance granulation efficiency, an organic granulating aid may be added. Examples of granulating aids include acrylic resin, copolymers of isobutylene and maleic anhydride, polyvinyl alcohol, polyethylene glycol, polyvinyl pyrrolidone, hydroxypropyl cellulose, methyl cellulose, cornstarch, gelatin, and lignin. The amount of the granulating agent added is preferably about five parts by weight or less on the basis of 100 parts by weight of a mixture of the sintering accelerator and the composite oxide having a spinel structure predominantly comprising lithium, manganese, and oxygen, more preferably two parts by weight or less.
A method for sintering the granules will next be described.
The granules containing the organic binder are degreased at about 300-550xc2x0 C. for about 10 minutes or more in air or in an oxygen-gas-containing atmosphere. The amount of residual carbon in the granules is preferably about 0.1% or less. The degreased granules are sintered at about 550-900xc2x0 C. for about one minute or more in air or in an oxygen-containing atmosphere.
Even when an organic granulating aid is not employed, the granules may be sintered and shrunk in air or in an oxygen-gas-containing atmosphere in the manner described above, to thereby produce dense secondary particles
When the thus-produced granules (including secondary particles) or the primary particles of the composite oxidexe2x80x94the surface layer of the granules or primary particles comprising a xcex2-MnO2 phasexe2x80x94are employed as the cathode electroactive material, the non-aqueous secondary cell comprising the substance has a high initial discharge capacity. In addition, the discharge capacity of the cell is barely reduced even when charging and discharging at 60xc2x0 C. is carried out repeatedly. In the case in which the granulated secondary particles have a size of about 20 xcexcm, the thickness of the surface layer comprising xcex2-MnO2 is about 0.02-0.22 xcexcm, preferably about 0.05-0.08 xcexcm.
The process for producing the cathode electroactive material of the present invention is characterized in that xcex-MnO2 comprised in the surface of the composite oxide having a spinel structure is converted into xcex2-MnO2 which is stable throughout the charging and discharging of the cell. This is brought about through heat treatment at a temperature of about 200xc2x0 C. or more but less than about 400xc2x0 C. When the heat treatment is carried out at temperatures lower than about 200xc2x0 C., xcex-MnO2 may not be converted into xcex2-MnO2, whereas when the heat treatment is carried out at about 400xc2x0 C. or higher, lithium contained in the composite oxide having a spinel structure diffuses on the surface of the oxide, and thus the surface cannot be stabilized. Diffusion of lithium on the surface of the oxide can be confirmed by the lattice constant of the oxide; i.e., when lithium diffuses on the surface of the oxide, the lattice constant of the oxide is larger than that of the oxide which is heat-treated at a temperature of about 200xc2x0 C. or higher but less than about 400xc2x0 C. The heat treatment may be carried out for at least five minutes.
In the present invention, particles of the composite oxide having a spinel structure comprising predominantly lithium, manganese, and oxygen may be subjected to acid treatment, which results in the development of pores of 30-400 xc3x85. When the composite oxide particles having such pores are employed as the cathode electroactive material, the current density of the surface of the substance may be locally reduced during charging and discharging, and thus the cycle characteristics of the cell may be remarkably enhanced.
In general, in order to enhance cell characteristics, side reactions of the cathode electroactive material must be suppressed, and thus it is desirable that the cathode electroactive material have a small specific surface area (about 1 m2/g or less). Surprisingly, although the cathode electroactive material of the present invention has a large specific surface area of about 1.5 m2/g or more, the secondary cell exhibits excellent characteristics as compared with a conventional cell.
A method for employing the cathode electroactive material of the present invention as a material for a positive electrode in a non-aqueous secondary cell will next be described.
The positive electrode material is produced through the following procedure: the cathode electroactive material, a conductivity-imparting agent such as carbon black or graphite, and a binder such as polyvinylidene fluoride are dissolved in a solvent (e.g., N-methylpyrrolidone) in appropriate proportions and kneaded; the resultant electrode paste is applied to a current-collecting material; and the paste-applied material is dried and then pressed using a roll press. The current-collecting material may be a known metallic current-collecting material such as aluminum, stainless steel, or titanium.
In the non-aqueous secondary cell of the present invention, an electrolytic salt contained in an electrolytic solution may be a known fluorine-containing lithium salt. For example, LiPF6, LiBF4, LiN(CF3SO2)2, LiAsF6, LiCF3SO31 or LiC4F9SO3 may be employed. The electrolytic solution of the non-aqueous secondary cell is produced by dissolving at least one species of the aforementioned known fluorine-containing lithium salts in a non-aqueous electrolytic solution. The non-aqueous solvent for the non-aqueous electrolytic solution is not particularly limited, so long as the solvent is chemically or electrochemically stable and aprotic. Examples of such solvents include carbonic acid esters such as dimethyl carbonate, propylene carbonate, ethylene carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, methyl butyl carbonate, diethyl carbonate, ethyl propyl carbonate, diisopropyl carbonate, dibutyl carbonate, 1,2-butylene carbonate, ethyl isopropyl carbonate, and ethyl butyl carbonate; oligoethers such as triethylene glycol methyl ether and tetraethylene glycol dimethyl ether; aliphatic esters such as methyl propionate and methyl formate; aromatic nitrites such as benzonitrile and tolunitrile; amides such as dimethylformamide; sulfoxides such as dimethyl sulfoxide; lactones such as y-butyrolactone; sulfur compounds such as sulfolane; N-vinylpyrrolidone; N-methylpyrrolidone; and phosphoric acid esters. Of these, in the present invention, carbonic acid esters, aliphatic esters, or ethers are preferable.
In the non-aqueous secondary cell of the present invention, the material for the negative electrode is not particularly limited, so long as it can reversibly occlude or release lithium ions. For example, the material may be metallic lithium, lithium alloy, carbon material (including graphite), or metallic chalcogen.
A method for evaluating electrode characteristics will next be described.
The cathode electroactive material, Vulcan XC-72 (product of Cabot Corp.) serving as a conductive material, and an ethylene tetrafluoride resin serving as a binder are mixed in proportions by weight of 50:34:16, and the resultant mixture is relaxed with toluene over 12 hours. The expanded mixture is applied onto a current-collecting material comprising aluminum expanded metal, and shaped at a pressure of about 2 t/cm2, and the toluene is then evaporated to thereby produce a positive electrode. The negative electrode is produced from lithium foil. Propylene carbonate and dimethyl carbonate are mixed in a ratio by volume of about 1:2, and LiPF6 is dissolved in the resultant mixture in a concentration of 1 mol/liter, to thereby produce an electrolytic solution. A separator made of polypropylene is employed. In order to prevent a micro short circuit due to dendrite formation in the negative electrode, silica fibrous filter paper QR-100 (product of Advantec Toyo Co.) serving as a reinforcing material is added. A 2016-type coin-shaped cell is produced from a combination of the positive electrode, the negative electrode, the electrolytic solution, the separator, and the reinforcing material. The thus-produced cell is subjected to a charging and discharging test of 500 cycles in a thermostatic chamber of 60xc2x0 C. Measurement conditions are as follows: constant-current-constant-voltage charging and constant-current discharging; charging or discharging rate IC (charging time: 2.5 hours); and scanning voltage 3.1-4.3 V.
The cathode electroactive material and the non-aqueous secondary cell comprising the substance of the present invention will next be described in detail by way of Examples and Comparative Examples, which should not be construed as limiting the invention thereto.
Measurement of the distribution of pores of 400 xc3x85 in the cathode electroactive material of the present invention was carried out as follows. A porosimeter (model: 2000WS, product of Carloelva) was employed as a measurement apparatus; the cathode electroactive material and mercury were maintained in vacuum for one hour (pre-treatment for measurement); and then mercury was introduced into the pores of the substance under pressure of 2,000 bar over 40 minutes and measurement was carried out.
The structure of the cathode electroactive material was confirmed through X-ray diffraction under the following conditions. X-ray source: CuKxcex1; output: 50 kV, 180 mA; slit: 1/2-1/2-0.15 mm; measurement method: 2xcex8/xcex8 method; measurement range: 20-90xc2x0; scanning rate: 5xc2x0/minute.